Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide systems and methods for overvoltage protection. Merely by way of example, some embodiments of the invention have been applied to LED lighting. But it would be recognized that the invention has a much broader range of applicability.
Conventional power conversion systems with primary-side regulation (PSR) and buck-boost mechanism have been widely used for light emitting diode (LED) lighting. FIG. 1 is a simplified diagram showing a conventional AC-to-DC power conversion system with primary-side regulation (PSR) and buck-boost mechanism for LED lighting. The AC-to-DC power conversion system 100 (e.g., a power converter) includes resistors 110 and 118, capacitors 112, 114, and 116, a pulse-width-modulation (PWM) controller 120, a switch 140, an inductive winding 142, and a diode 144.
For example, the AC-to-DC power conversion system 100 includes only one inductive winding (e.g., the inductive winding 142). In another example, the pulse-width-modulation (PWM) controller 120 includes a terminal 122 (e.g., pin VDD), a terminal 124 (e.g., pin COMP), a terminal 126 (e.g., pin GATE), a terminal 128 (e.g., pin CS), and a terminal 130 (e.g., pin GND).
As shown in FIG. 1, an AC input voltage 150 (e.g., VAC) is received and processed with full-wave rectification to generate a rectified voltage 152 (e.g., Vin). For example, the rectified voltage 152 does not fall below 0 volt. In another example, the rectified voltage 152 charges the capacitor 112 (e.g., C2) through the resistor 110 (e.g., R2) in order to increase a voltage 154 in magnitude. The voltage 154 is received by the PWM controller 120 through the terminal 122. If the voltage 154 becomes larger than an undervoltage-lockout (UVLO) threshold, the PWM controller 120 starts the normal operation.
Under normal operation, the PWM controller 120 generates a drive signal 156 with pulse-width modulation. For example, the PWM controller 120, after detecting the end of a demagnetization process, uses an error amplifier as part of the PWM controller 120 to control charging and discharging of the capacitor 116 (e.g., C3) through the terminal 124. In another example, the resistor 118 is used to sense the current flowing through the inductive winding 142 and to provide the sensing voltage to the PWM controller 120 through the terminal 128. In response, the PWM controller 120 processes the sensing voltage on a cycle-by-cycle basis by sampling the peak magnitude of the sensing voltage and sending the sampled peak magnitude to the error amplifier as part of the PWM controller 120.
The PWM controller 120 outputs the drive signal 156 to the switch 140 through the terminal 126. For example, the drive signal 156 has a frequency and also a duty cycle. In another example, the drive signal 156 opens (e.g., turns off) and closes (e.g., turns on) the switch 140. Additionally, the capacitor 114 (e.g., C5) is used to support an output voltage 160 (e.g., Vo) of the power conversion system 100. For example, the power conversion system 100 provides a constant output current to one or more light emitting diodes (LEDs) 190. In another example, the inductive winding 142 includes winding terminals 141 and 143, and the diode 144 includes diode terminals 145 and 147. For example, the winding terminal 143 is coupled to the diode terminal 145. In another example, a voltage difference between the diode terminal 147 and the winding terminal 141 is equal to an output voltage 160 (e.g., Vo) of the power converter 100.
As shown in FIG. 1, the AC-to-DC power conversion system 100 with primary-side regulation (PSR) and buck-boost mechanism includes only one inductive winding (e.g., the inductive winding 142). The AC-to-DC power conversion system 100 includes a conventional single-inductive-winding buck-boost structure. The conventional single-inductive-winding buck-boost structure often has certain advantages as well as some weaknesses in comparison with conventional buck-boost structures that include two or more inductive windings. For example, the conventional single-inductive-winding buck-boost structure can reduce external bill of materials (BOM) and also cost of the power conversion system. In another example, the conventional single-inductive-winding buck-boost structure does not include a secondary winding, so the conventional single-inductive-winding buck-boost structure usually cannot directly measure and/or precisely determine a magnitude of the output voltage. This lack of precise determination of the output voltage magnitude usually causes the conventional power conversion system not able to timely turn off the switch and/or effectively perform the function of overvoltage protection (OVP). As a result, the output capacitor (e.g., the capacitor 114) can be damaged by the excessive output voltage.
Hence, it is highly desirable to improve techniques for overvoltage protection of a power conversion system.